High performance frequency reference design using a PWM

A PWM circuit works by making a square wave with a variable on-to-off ratio; the average on time may be varied from zero to 100%. In this manner, a variable amount of power is transferred to the load. The main advantage of a PWM circuit over a resistive power controller is the efficiency. At a 50% level the PWM will use about 50% of full power -- almost all of which is transferred to the load. A resistive controller at 50% load power would consume about 71% of full power with some 50% of the power going to the load and the other 21% being wasted to heat the dropping resistor. Load efficiency is almost always a critical factor in alternative energy systems.

With today's increasingly crowded communications spectrum, the need for high performance frequency reference is increasing. High temperature stability is crucial for the performance of many types of telecom equipment — in both wireless and fibre optics. The ovenised oscillator and heater based platforms are attractive answers to these and other applications requiring a high performance to cost relationship.

Timing

In the design of a circuit for PWM control, a 555 timer can be used on its own as a modulator. Unfortunately it is not possible to vary the modulation from zero to 100%, which limits its usefulness in this application. However, the 555 timer can be used as a sawtooth oscillator to set the modulation frequency.

The symmetry of the waveforms and duty cycle can be adjusted with the external timing resistors R1 and variable resistor U3a, and capacitor C1 to a 555 time as shown in figure 4. The charge and discharge of C1 to a 555 timer will determine this waveform timing. The magnitude of the triangle waveform is at 1/3V supply, therefore the rise and fall portions of the sawtooth are between the 1/3V and 2/3V supply window.

Figure 4 shows the threshold and trigger terminals of the 555 timer which are tied together to form a common input to a comparator, the device forming an inverting Schmitt trigger with: VTL= (1/3)Vcc, VTH = (2/3)Vcc, VOL= 0, VOH = Vcc. Calculation for the rise and fall time of the sawtooth are: Rise Time, T = 0.7 (R1 C1), Fall Time, T = 0.7 (U3a C1). Note: U3a = 50k variable resistor (X9259 Xicor Digital Potentiometer). The design formula for the frequency of the pulses is: F = 1.44/(R1+ U3a) C1.

The standard 555 oscillator circuit has the disadvantage that it has a duty cycle bigger than 50%. Yet by the use of a diode D1 in parallel with the discharge resistor U3a shown in figure 4, it is possible to charge C1 only through R1 resulting in an accurate mark-space adjustment which can be defined by the size of the resistors R1 and variable resistor U3a.

Comparator

In figure 3 the +ve terminal of the comparator is fed by a controlled voltage, which is directly associated with the oven temperature. This voltage will sweep across the dynamic range of the Sawtooth. From this the threshold for the modulation is adjusted, and the timing characteristics of the modulation scheme are dynamically altered to provide the ideal duty cycle for the system. In this application the controlled voltage is derived from a temperature sensor which could be controlling an ovenised oscillator or wave guide array.

The High and Low times of each pulse can be calculated from: High Time = (V Threshold) (Rise Time) and Low Time = (V Threshold) (Fall Time). The duty cycle of the waveform, usually expressed as a percentage, is given by: Duty Cycle = High Time/Pulse Period Time.

Error amplifier

The error amplifier is used to integrate the difference between the reference input at the +ve terminal and the feedback signal (temperature sensor) at the --ve terminal. Its output voltage will go to the exact voltage required by the PWM block to generate the proper duty cycle corresponding to the desired output. The main purpose of the error amplifier is to respond to input signal changes, but it also compensates other variables inside the feedback loop.

Systems such as oven controllers and Paltier controllers will have to deal with variations in the ambient temperature in addition to temperatures exhibited by the system itself.

Figure 5 shows a schematic of a temperature sensor and signal processing section that could be used in applications such as ovenised oscillators and wave guide array or Paltier coolant in fibre optic applications.

The thermostat and the resistor form a bridge, which senses and ultimately controls the temperature of the oven. The gain adjustment potentiometer corrects the initial inaccuracies produced from error amplifier voltage offset, tolerances of the resistors and the thermostat.

The offset trimming resistor U3c sets the bias to the non-inverting input of the error amplifier to the middle of its common mode voltage range, whilst also helping to trim the output of the amplifier to a set point for the required operating temperature.

The PWM circuit converts the error amplifier output into a variable duty cycle drive signal. The error amplifier's function is to take a scaled version of the output, V- = -Vo(U3d/R4), compare it against the reference voltage V+ = Vref, and adjust Vo. Vo = (-R1/R2)Vref. As temperature changes a scaled version of the change will appear at the Vo.

Power transistor

Q1 is the power switch, receiving the modulated pulse width voltage at its gate terminal and switching the load current on and off through the source-drain current path.

When Q1 is on, it provides a ground path for the load, indicated as I1. When Q1 is off, current still flows because of the oven inductance, however this time current I2 will flow through the diode, which is now forward biased.

In order to simplify the setting of all the trimming levels for various parts of the circuit, a quad 50k? digital controlled potentiometer type X9259 is used to adjust the PWM duty cycle and the EA offset and gain adjustment.

Figure 7 shows that all four potentiometers on the X9259 exhibit the same resistive trend when tracking over temperature and that the total drift is relatively small, especially at the higher end.

Oven application

If we consider an application where an encapsulated ceramic hybrid unit measuring around 3x2cm, has a VCO with oven wrapped around it. The oven is made of a thin layer of copper and is to generate a constant temperature somewhere in the region of 70 to 80°C, depending on the crystal as the optimum operating point for each crystal is different.

At the production and burning out process the temperature would be swept through the 70 to 80°C window and the output frequency of the unit would be measured until the optimum frequency is achieved.

The range of frequencies are carrier dependent and could vary from 10 to 100MHz, so each unit would present only a specific frequency which is required for a given base station type.

The output frequency from this unit would then be used as a very accurate reference into synthesisers.

X9259 is used in the analogue circuit to adjust both the current and the slope gradient, which is the final trimming. The oven power rating is 2.5W @ 5V giving a maximum current of 2A.

This circuit could be modified to cater for applications requiring bidirectional current control such as the Paltier coolant used in fibre- optics.

If you have any comments to make about this article, or perhaps you require more information from the author,please email awinning@cmp-europe.com
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